Nanoprisms and method of making them

ABSTRACT

The invention is a novel photo-induced method for converting large quantities of silver nanospheres into nanoprisms, the nanoprisms formed by this method and applications in which the nanoprisms are useful. Significantly, this light driven process results in a colloid with a unique set of optical properties that directly relate to the nanoprism shape of the particles. Theoretical calculations coupled with experimental observations allow for the assignment of the nanoprism plasmon bands and the first identification of two distinct quadrupole plasmon resonances for a nanoparticle. Finally, unlike the spherical particles from which they derive and which Rayleigh light scatter in the blue, these nanoprisms exhibit scattering in the red, permitting multicolor diagnostic labels based not only on nanoparticle composition and size but also on shape.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 USC § 119(e) of U.S.Provisional Application No. 60/325,293, filed Sep. 26, 2001, which isincorporated herein in its entirety by this reference.

STATEMENT OF GOVERNMENTAL INTERESTS

This invention was made with government support under National ScienceFoundation grant CHE-9871903 and Army Research Office grantDAAG55-97-1-0133. The government has certain rights in this invention.

FIELD OF THE INVENTION

The invention resides in the field of nanoprisms having unique opticalproperties produced by a photo-induced method.

BACKGROUND OF THE INVENTION

Size is an important parameter in nanoscale materials that can providecontrol over many of their physical and chemical properties, includingluminescence, conductivity, and catalytic activity. Over the pastcentury, colloid chemists have gained excellent control over particlesize for several spherical metal and semiconductor compositions. Thischemical control over particle size has led to the discovery of quantumconfinement in colloidal nanocrystals and their exploitation as probesin biological diagnostic applications, LED materials, lasers, and Ramanspectroscopy enhancing materials. In contrast, the challenge ofsynthetically controlling particle shape has been met with limitedsuccess. Nevertheless, some physical and solid-state chemical depositionmethods have been developed for making semiconductor and metalnanowires, nanobelts, and dots, and there are now a variety of methodsfor making rods with somewhat controllable aspect ratios usingelectrochemical and membrane-templated syntheses.

Less is known with respect to solution synthetic methods fornon-spherical particles such as triangles or cubes. However, methods doexist for making colloidal samples of Pt cubes and pyramids (Ahmandi etal., Science 272:1924 (1996)), and PbSe, CdS, and Ni triangles (Fendleret al., J. Am. Chem Soc. 122:4631 (2000), Pinna et al., Ad. Mater.13:261 (2001), Klasu et al., Proc. Natl. Acad. Sci U.S.A. 96:13611(1999)). Promising recent work has resulted in methods for synthesizingBaCrO₄, CdSe and Co nanorods and distributions of arrow-, teardrop-, andtetrapod-shaped CdSe nanocrystals (Li et al., Nature 402:393 (1999),Peng et al., Nature 404:59 (2000), Puntes et al., Science 291:2115(2001), Manna et al., J. Am. Chem. Soc. 122:12700 (2000)). All of thesesolution methods are based on thermal processes, and in most cases, withthe exception of rods, yield relatively small quantities of the desiredparticle shape. However, much like particle size-control in nanoscalematerials led to the discovery of new and important fundamental scienceand technological applications in diagnostics, optics, catalysis, andelectronics, synthetic methods that lead to control over particle shapecan be expected to lead to important fundamental as well astechnological advances. Therefore, the development of bulk solutionsynthetic methods that offer control over particle shape is of paramountimportance if the full potential of these novel materials is to berealized.

SUMMARY OF THE INVENTION

One embodiment of the invention provides a method of forming nanoprismsby exposing a suspension of non-crystalline silver particles to lighthaving a wavelength of less than about 700 nm to form silver crystals.The method uses a wavelength of light between about 350 nm and about 700nm. The suspension may comprise a reducing agent, a stabilizing agent,and a surfactant and the light exposure may be intermittent or continueover hours, days, weeks or longer. The suspension may also be stablymaintained in the absence of light having a wavelength of less thanabout 700 nm.

Another embodiment of the present invention is a method of formingnanoprisms by fragmenting silver nanoparticles in suspension by exposureto light, growing silver nanoprisms by continued exposure to light andthen terminating the growth of the silver nanoprisms. Preferably, thelight has a wavelength of between about 350 nm and about 700 nm and thegrowth is continued for a period of greater than 50 hours. The growthcan be terminated by preventing further exposure of the suspension tolight or by consuming the silver nanoparticles feeding the growth. Thesuspension may comprise a reducing agent, a stabilizing agent, and asurfactant and the light exposure may be intermittent or continue overhours, days, weeks or longer. The suspension may also be stablymaintained in the absence of light having a wavelength of less thanabout 700 nm.

Another embodiment of the invention is a silver nanoprism comprising asingle silver crystal having a lattice spacing of 1.44 Å and an edgelength of between about 10 nm and about 60 nm. The nanoprism typicallyhas a triangular shape with an atomically flat top and bottom. Thenanoprism may also have an in-plane dipole plasmon resonance of 770 nmor an out-of-plane dipole plasmon resonance of 410 nm, or an in-planequadrupole resonance of 470 nm, or an out-of-plane quadrupole resonanceof 340 nm or any combination of these characteristics. A furtherembodiment of the present invention is a silver nanoprism having a tipof the prism is removed. These nanoprisms, may exhibit Rayleighscattering in the red.

Another embodiment of the present invention is a nanoparticle labelbased on a silver nanoprism. The label has a triangular shape with anatomically flat top and bottom. The nanoprism label may also have anin-plane dipole plasmon resonance of 770 nm or an out-of-plane dipoleplasmon resonance of 410 nm, or an in-plane quadrupole resonance of 470nm, or an out-of-plane quadrupole resonance of 340 nm or any combinationof these characteristics. A further embodiment of the present inventionis a silver nanoprism having a tip of the prism is removed. Thesenanoprisms, may exhibit Rayleigh scattering in the red.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the U.S. Patent and TrademarkOffice upon request and payment of the necessay fee.

FIG. 1 shows time-dependent UV-vis spectra showing the conversion ofsilver nanospheres to nanoprisms: (A) before irradiation, (B) after 40 hof irradiation, (C) after 55 h of irradiation, (D) after 70 h ofirradiation. The inset shows the extinction profile at 670 nm as afunction of time.

FIG. 2 shows TEM images (reverse print) mapping the morphology changesas a function of irradiation time. (A) before irradiation, (B) after 40h of irradiation, (C) after 55 h of irradiation, (D) after 70 hr ofirradiation. The scale bar is 200 nm for all four images.

FIG. 3 shows (A) EELS mapping analysis showing the flat-top morphologyof the silver nanoprisms; the inset shows the EELS intensity over theline scan. (B) Stacks of silver nanoprisms assembled in a top-to-basemanner on a carbon film-coated copper grid.

FIG. 4 shows electron diffraction analysis of individual silvernanoprisms. The spot array, diagnostic of a hexagonal structure, is fromthe [111] orientation of an individual silver nanoprism lying flat onthe substrate with its top perpendicular to the electron beam. Based on3 zone axis analysis (not shown), the crystal structure of the silvernanoprism was determined to be an fcc structure. The intense spots inthe [111] zone axis are allowed {220} Bragg reflections (e.g. circledspot) and the sharp weak spot in the center of the triangles formed bythe strong spots is indexed as ⅓ {{overscore (4)}22} (e.g. boxed spot).

FIG. 5 shows DDA simulations of the orientation averaged extinctionefficiency spectra of two silver nanoprisms in water: (A) a perfectlytriangular nanoprism, 8512 dipoles are used in the calculations, and (B)a truncated triangular nanoprism, 7920 dipoles are used for thecalculation. Dielectric constant data are taken from E. D. Palik,Handbook of Optical Constants of Solids (Academic Press, New York,1985).

FIG. 6 shows Rayleigh light scattering of particles deposited on a glassslide. The slide is used as a planar waveguide, which is illuminatedwith a Tungsten source. The image was taken with a digital camera.

FIG. 7 is a schematic depiction of the formation and growth of thenanoprisms from nanospheres.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a unique photo-induced method for synthesizinglarge quantities of metal nanoprisms in high yield in the form of acolloidal suspension. Importantly, this photo-mediated route has led toa colloid with a unique set of optical properties that directly relateto the shape control afforded by this novel preparatory method.

In one embodiment of the invention, nanoprisms are formed by exposing asuspension of spherical silver particles to light having a wavelength ofless than 700 nm. The reaction that transforms the spherical particlesinto nanoprisms is initiated by the exposure to light. Exposure to lightin the range of about 350 nm to about 700 nm, such as the light from a40 W fluorescent bulb or tube, is preferred. The reaction is notinitiated in the dark or by exposure to light having a wavelengthgreater than about 700 nm (near-IR light) and the starting suspension isstable as spherical particles for at least two months when protectedfrom light. Therefore, the reaction which results in nanoprisms can beselectively turned on or off simply by controlling the exposure of thecolloid to light of the appropriate wavelength.

The silver salt precursor forming the silver source in suspension may beany silver salt capable of dissolution in the selected suspensionmedium. For example, AgNO₃ and AgClO₄ perform comparably as a silversource for the suspension in an aqueous solution. Preferably AgNO₃ isused in a 0.1 mM aqueous solution. Additionally, a reducing agent isoptionally added to the suspension to aid in formation of thesuspension. Preferably, the reducing agent is NaBH₄ in a finalconcentration of 0.5 mM that may be added in dropwise increments.

The surfactant used to form the suspension of nanospheres may varywidely in concentration without affecting the extent of the conversionof nanospheres to nanoprisms. However, the reaction rate is affected bysurfactant and provides an additional means of controlling theconversion reaction based on the conversion rate. Preferably, trisodiumcitrate is present as a surfactant in the suspension of silvernanospheres and bis(p-sulfonatophenyl) phenylphosphine dihydrate (BSPP)is added to the suspension as a particle stabilizing agent. Although thenanoprisms are formed over the entire range of surfactant concentration,the rate of the conversion reaction decreases as a function ofincreasing the ratio of BSPP to citrate over a range of about 0.01 toabout 1. The most rapid conversion rate is obtained at a BSPP to citrateratio of 0.3:1. Thus, the reaction rate may be optimized by varying thesurfactant concentration and the ratio of the surfactant to astabilizing agent added to the suspension.

After formation of the suspension and exposure to light of the correctwavelength the solution initially turns yellow, characteristic of thespherical particles, but over a time period that can be controlled bysuspension characteristics and light exposure, the suspension turnsgreen and then finally blue. As shown in FIG. 1, using UV-visspectroscopy, the characteristic surface plasmon band for the sphericalparticles at λ_(max)=400 nm decreases in intensity with a concomitantgrowth of three new bands with λ_(max)=335 (weak), 470 (mediate), 670 nm(strong) respectively. As the conversion of the nanospheres tonanoprisms nears completion, the band at 400-nm completely disappears.

Using spectroscopy and electron microscopy, three distinct stages in theconversion of nanospheres to nanoprisms have been identified includinginduction, growth, and termination, FIG. 1 (inset). Detailed TEM studiesreveal that during the induction period, extremely small sphericalsilver clusters (about 2 to about 4 nm) are formed which are not presentin the solution containing the initial spherical particles as shown inFIG. 2B. These silver clusters form from either fragmentation ordissolution of the larger particles. Noteably, photo-inducedfragmentation of silver nanoparticles (visible laser at 532 nm) has beenobserved by Hartland and coworkers (Kamat et al., J. Phys. Chem. B102:3123 (1998)). Nanoprisms form concurrently with the formation ofthese small clusters. As depicted in FIG. 7, the silver nanoprisms thenact as seeds and grow as the small spherical crystals are digested. Oncethe spherical particles and small nanoclusters are consumed, thereaction terminates. The conversion process can be arrested at any pointby stopping the light exposure, thereby providing a means of controlover the shape and size of the nanoparticles in suspension. Thephoto-induced fragmentation of silver particle precursors into smallclusters makes the use of light an efficient way to control the growthof the silver nanoprisms. It is important to note that other researchershave used visible lasers, UV, or γ irradiation to prepare sphericalsilver nanocrystals from silver salts in the presence of organicreducing agents (Henglein, Langmuir 17:2329 (2001), Prochazka et al.,Anal. Chem. 69:5103 (1997)). Typically, photo-induced reductionmechanisms are invoked to describe such processes, however, thesemechanisms are in contrast with the growth mechanism proposed herein forthe silver nanoprisms where the spherical silver particle precursorsbegin in the reduced state and are exclusively transformed intonanoprisms via the light-induced fragmentation process.

As shown by transmission electron microscopy (TEM) correlated with thetime-dependent spectroscopic observations, the silver nanoprisms evolvefrom the initial spherical nanoparticles. FIGS. 2A-D shows the initialspherical silver particles (8.0±1.7 nm) are converted over the reactionperiod to prismatic structures, which appear in two dimensions astriangles. During the initial stages of growth, both spheres and prismscan be observed as shown in FIG. 2B. The latter exhibit edge lengthsbetween about 10 nm and about 60 nm. As shown in FIG. 2C, both the sizeand population of the silver prisms increase with time with aconcomitant decrease in the number of spherical particles. The reactionproceeds to completion and nearly all of the initial spheres (>99%) areconverted to the prismatic structures having an edge length of about 100nm, FIG. 2D.

Another embodiment of the present invention includes the nanoprismsformed by the light-induced photoconversion processes described above.TEM images and electron energy loss spectroscopy analysis (EELS) showthat the particles formed in this reaction are indeed silver nanoprismsand not triangular tetrahedra. FIG. 3 shows that each nanoprism has aflat top and bottom (FIG. 3A), and the triangular thickness fringesexpected for triangular tetrahedra are not observed in the TEM. Upon theevaporation of solvent, the silver nanoprisms assemble into “stacks” onthe TEM grids (FIG. 3B) allowing precise measurement of their thickness(15.6±1.4 nm). These stacks appear as nanorods in the two-dimensionalTEM images, but tilting experiments confirm that they are nanoprisms.Significantly, each nanoprism is a single crystal with a lattice spacingof about 1.44 Å, as evidenced by electron diffraction analysis shown inFIG. 4. Detailed TEM investigations (tilting diffraction with 3 zoneaxis) of individual silver nanoprisms show that the 1.44 Å latticespacing corresponds to Bragg diffraction from their {220} crystal faces(face-centered cubic). Therefore, the top crystal face of each nanoprismmust be (111). Interestingly, an additional set of relatively weak spotsin the diffraction pattern, corresponding to 1 {422} with a 2.5 Åspacing is also observed. These weak diffraction spots derive from thelocal hexagonal-like structure observable only for a silver or goldsample that is atomically flat. This is consistent with the structuralcharacterization of these novel particles as thin nanoprisms withatomically flat tops and bottoms.

The shape and dimensions of the nanoprism depicted in FIG. 5A areaverage representations of the triangular prisms observed in the TEMimages shown in FIG. 2D. Because the nanoprisms have a triangular shape,the large structural anisotropy substantially influences their opticalproperties (i.e. light-absorption, scattering and SERS). To characterizethe extinction spectrum shown in FIG. 1D, Maxwell's equations for lightinteracting with a triangular prism were solved using a finiteelement-based method known as the Discrete Dipole Approximation (Yang etal., J. Chem. Phys. 103:869 (1995)). Comparing FIGS. 1D and 5A, threebands are observed which qualitatively match the wavelengths of themeasured spectra. Examination of the induced polarizations associatedwith these peaks indicates that the 770 nm peak is the in-plane dipoleplasmon resonance, while 470 nm is the in-plane quadrupole resonance,and 340 nm is the out-of-plane quadrupole resonance. The out-of-planedipole resonance 410 nm and is sufficiently weak and broad that it isbarely discernable as a shoulder on the 470 nm peak. Additionalcalculations indicate that the peak at 770 nm is very sensitive to thesharpness of the tips on the triangles as shown in FIG. 5B. For example,if an about 12 nm region at each tip of a prism is removed, the longwavelength resonance at 770 nm for the perfect prism shifts to 670 nmwithout significantly changing the other resonances. Referring to FIG.2B, TEM shows that about 20% of the nanoprisms are truncated. Therefore,these calculations not only allow identification of the importantfeatures in the spectrum of the nanoprisms but also the relationshipbetween particle shape and frequency of the bands that make up theirspectra.

The optical properties of these novel nanostructures are striking. Forexample, these nanoprisms provide the first observation of two distinctquadrupole plasmon resonances for a nanoparticle. Additionally, unlikethe spherical particles from which they are derived, and which scatterlight in the blue, as shown in FIG. 6A, the nanoprisms of the presentinvention exhibit Rayleigh scattering in the red. Light scattering ofmetal nanoparticles probes has already been exploited in the developmentof many biodiagnostic applications and although conventional sphericalparticles made of gold or silver do not scatter in the red, theirscattering properties can be tailored by adjusting their size andcomposition as shown in FIGS. 6B-E. Therefore, these novel nanoprismsand their unusual optical properties permit development of multicolorlabels based on nanoparticle composition, size and shape.

The nanoparticles can be used as new diagnostic labels, lighting up whentarget DNA is present. Biodetectors incorporating nanoprisms can be usedto quickly, easily and accurately detect biological molecules as well asa wide range of genetic and pathogenic diseases, from genetic markersfor cancer and neurodegenerative diseases to HIV and sexuallytransmitted diseases.

1. A silver nanoprism comprising a single silver crystal.
 2. Thenanoprism of claim 1, having an edge length of between about 10 nm andabout 60 nm.
 3. The nanoprism of claim 1, having a triangular shape withan atomically flat top and bottom.
 4. The nanoprism of claim 1, havingan in-plane dipole plasmon resonance of 770 nm.
 5. The nanoprism ofclaim 1, having an out-of-plane dipole plasmon resonance of 410 nm. 6.The nanoprism of claim 1, having an in-plane quadrupole resonance of 470nm.
 7. The nanoprism of claim 1, having an out-of-plane quadrupoleresonance of 340 nm.
 8. The nanoprism of claim 1, wherein a tip of theprism is removed.
 9. The nanoprism of claim 8, wherein the removed tipis about 12 nm in height.
 10. The nanoprism of claim 1, exhibitingRayleigh scattering in the red.
 11. A nanoparticle label comprising asingle silver crystal nanoprism.
 12. The nanoparticle label of claim 11,wherein the nanoprism has a triangular shape with an atomically flat topand bottom.
 13. The nanoparticle label of claim 11, wherein thenanoprism has an in-plane dipole plasmon resonance of 770 nm.
 14. Thenanoparticle label of claim 11, wherein the nanoprism has anout-of-plane dipole plasmon resonance of 410 nm.
 15. The nanoparticlelabel of claim 11, wherein the nanoprism has an in-plane quadrupoleresonance of 470 nm.
 16. The nanoparticle label of claim 11, wherein thenanoprism has an out-of-plane quadrupole resonance of 340 nm.
 17. Thenanoparticle label of claim 11, wherein a tip of the nanoprism isremoved.
 18. The nanoparticle label of claim 17, wherein the removed tipis about 12 nm in height.
 19. The nanoparticle label of claim 11,exhibiting Rayleigh scattering in the red.
 20. A silver nanoprism havingan in-plane quadrupole resonance of 470 nm and an out-of-planequadrupole resonance of 340 nm.
 21. The nanoprism of claim 20, having anatomically flat top and bottom and an edge length of between about 10 nmand about 60 nm.
 22. A silver nanoprism displaying both in-planequadrupole resonance and out-of-plane quadrupole resonance.